1. Field of the Invention
The present invention relates to a magnetic sensor and a magnetic recording and reproducing apparatus having a magnetic head including the magnetic sensor.
2. Description of the Related Art
Magnetic recording density has increased significantly since the advent of a giant magnetoresistive head (GMR head) that utilizes a giant magnetoresistive effect (GMR effect). A GMR element includes a stacked film (what is called a spin valve film) having a sandwich structure comprising a ferromagnetic layer, a nonmagnetic layer, and a ferromagnetic layer. The GMR element applies an exchange bias to one of the ferromagnetic layers to fix magnetization thereof, while allowing the magnetization direction of the other ferromagnetic layer to be changed by an external field. The GMR element thus detects a change in the relative angle between the magnetization directions of the two ferromagnetic layers, as a change in resistance value. A CIP-GMR element and a CPP-GMR element have been developed so far: the CIP-GMR element allows a current to flow in the plane of the spin valve film to detect a change in resistance; the CPP-GMR element allows a current to flow perpendicularly to the plane of the spin valve film to detect a change in resistance. Both CIP-GMR and CPP-GMR elements can provide a magnetoresistive ratio (MR ratio) of several percents. Thus, both elements are expected to achieve a recording density of up to about 200 Gbit/inch2.
Efforts have been made to develop a TMR element that utilizes a tunnel magnetoresistive effect (TMR effect), in order to achieve higher magnetic recording density. The TMR element includes a stacked film comprising a ferromagnetic layer, an insulating layer, and a ferromagnetic layer. The TMR element applies a voltage to between the ferromagnetic layers to allow a tunnel current to pass through the insulating layer. The TMR element detects a change in relative magnetization angle as a change in tunnel resistance value on the basis of a change in the magnitude of the tunnel current associated with the magnetization directions in the upper and lower ferromagnetic layers. The TMR element can provide an MR ratio of up to about 100%. The TMR element has a higher MR ratio than the GMR element and thus offers a high signal voltage. However, the TMR element provides not only a high level of real signal component but also a high level of noise component resulting from shot noise. Thus, the TMR element disadvantageously fails to achieve a high signal-to-noise ratio (SNR). The shot noise is caused by fluctuation of a current resulting from the irregular passage of electrons through a tunnel barrier and increases in proportion to the square root of tunnel resistance. In order to obtain a required signal voltage while suppressing the shot noise, the tunnel insulating layer should be made thinner to reduce the tunnel resistance. The element needs to be as small as a recording bit in order to deal with an increased recording density. It is thus necessary to reduce a junction resistance or a thickness of the tunnel insulating layer. With a recording density of 300 Gbit/inch2, the junction resistance needs to be at most 1 Ω·cm2. For an Al—O (aluminum oxide) or an Mg—O (magnesium oxide) tunnel insulating layer, the film thickness must be equal to two atomic layers. A thinner tunnel insulating layer is likely to short the circuit between the upper and lower ferromagnetic layers, thus lowering the MR ratio. This makes it markedly difficult to produce a reliable element. This is why the limit on the recording density achievable with the TMR element would be estimated at about 300 Gbit/inch2.
All the above elements utilize the magnetoresistive effect (MR effect) in a broad sense. In recent years, magnetic white noise has been a problem for all these MR elements. In contrast to electric noise such as the shot noise described above, the magnetic white noise is caused by thermal fluctuation of microscopic magnetization. The magnetic white noise thus becomes predominant as the size of the MR element is reduced. The magnetic white noise is expected to be more marked than the electric noise in elements that can achieve a recording density of 200 to 300 Gbpsi. To avoid the magnetic white noise and to further improve the magnetic recording density, a novel magnetic sensor needs to be developed which operates on the basis of a principle different from the one for the conventional elements.
On the other hand, a spin-wave oscillator has been proposed which is an example of applying motion of magnetization which occurs when a current flows perpendicularly to the plane of a three-layer structure comprising a first ferromagnetic layer, a nonmagnetic layer, and a second ferromagnetic layer (see Physical Review B. Volume 54, 9353 (1996)). The first ferromagnetic layer has magnetization pinned in a certain direction. Magnetization can rotate freely in the second ferromagnetic layer. When a current is supplied perpendicularly to the film plane, electrons flowing through the first ferromagnetic layer are spin-polarized. The resultant spin-polarized current is injected into the second ferromagnetic layer. The spins of the electrons interact with the magnetization in the second ferromagnetic layer to excite a spin wave. However, no technique for applying such a spin-wave oscillator to a magnetic sensor or the like is known.